1. Field of the Invention
This invention relates to semiconductor devices and more particularly to protection of semiconductor devices from problems attributable to excess energy.
2. Description of Related Art
Static charge can build up in integrated circuits, such as MOSFET semiconductor devices, to unacceptable levels in normal use. When the charge reaches a critical level the static charge is discharged in the integrated circuit, in an phenomenon known as ElectroStatic Discharge (ESD). The voltages reached during an ESD discharge event range as high as 1K Volts. Such voltages can damage the semiconductor circuits including the semiconductor MOSFET devices, or the metal contacts associated with the devices due to overheating caused by the excess heat the device dissipates during and ESD event. The amount of power which is converted into overheating is proportional to the square of the voltage reached; and the voltage reached is dependent on the breakdown voltage of the affected device.
Thick field oxide (FOX) devices have been used in input protection schemes also, but the disadvantage they have is high turn on voltage, usually larger than 12 Volts
U.S. Pat. No. 4,821,096 of Maloney for "Excess Energy Protection Device" shows ESD protection devices for MOSFET devices, using PMOS devices as protection devices, but there is no suggestion of using a series of MOS devices to modify the protection device.
The Maloney patent describes the background of ESD, pointing out that it is known that by coupling to the protected device, devices which have lower breakdown voltages, the power dissipated will be decreased, reducing the effect of heat in device failure. By dissipating the excess energy through a protecting device, the active device is not exposed to voltage levels which can cause failure. Certain types of excess energy events can adversely affect integrated circuits. Integrated circuits can be affected by other energy events from other sources, for example, voltage and current surges. These events create excess electrical energy to be dissipated within the circuits. The energy discharge path is from an input/output pad to ground and can involve the initial stages of the integrated circuit. If the excess energy event exceeds the oxide breakdown voltage of a device in the initial stage, oxide breakdown, damage and even failure result. Additionally, metal/silicon contacts and PN junctions may be thermally damaged as a result of the heat dissipated from the excess energy event.
To limit the impact of excess energy events, protection devices are inserted between the pad and the initial stage of the integrated circuit.
One protection device consists of a pair of diodes coupled to the input. One diode is coupled to a high power supply voltage level and the other is coupled to a low power supply voltage level or ground. A disadvantage of such a protection device is the excessively high voltage reference coupled to the first transistor.
A second protection device consists of a grounded gate n-channel transistor coupled through a resistor to a pad. This device is not always effective in CMOS applications. Studies have shown that the resistor fails or that n-channel devices fail during high energy events because of alloy spiking of the N+ junction. In N-MOS processing, this defect can be overcome by utilizing deep N+ junctions.
However, in CMOS processing shallow N+ junctions are often employed. As a result, the alloy spiking causes failure of N-channel protection devices. This defect can be overcome by enlarging the contact to gate spacing, but this requires additional silicon area which adds to the size of the integrated circuit.
U.S. Pat. No. 4,527,213 of Ariizumi for "Semiconductor Integrated Circuit Device with Circuits for Protecting an Input Section Against an External Surge" does not use a series of MOS devices to modify the gate voltage of the protection device.
U.S. Pat. No. 3,407,339 of Booher for "Voltage Protection Device Utilizing a Field Effect Transistor" uses a series of field effect transistors with drains coupled to gates for limiting the input voltage. However, Booher does not use this series of transistors to modify the gate voltage of the protection device. This type of protection circuit is not effective for MOS devices avoiding excessive static charge damage, because the excessive static charges must pass through several transistors from the input pad to ground.